1. Field of the Invention
The present invention relates to an adsorbent for adsorbing fuel vapors. The adsorbent adsorbs fuel vapors which leak from inlet systems of internal combustion engines, and emits them when inlet air is suctioned.
2. Description of the Related Art
Air polluting sources exhausted from internal combustion engines, such as gasoline engines and diesel engines, can be roughly divided into exhaust gases and fuel vapors.
The fuel vapors dissipate to air through inlet systems when internal combustion engines are stopped. As means for inhibiting the fuel vapors from dissipating, the following method is available. For example, a member for storing the fuel vapors is disposed in an inlet system of an internal combustion engine. The storage member temporarily adsorbs the fuel vapors which generate when the internal combustion engine is stopped. The storage member emits the fuel vapors to the inlet ports of the internal combustion engine when the internal combustion engine is driven. Thus, the fuel vapors are burned to purify. In this instance, the storage member is made generally by fixing an adsorbent on filters.
When internal combustion engines are stopped, the fuel vapors are filtered and adsorbed by adsorbents used as the member for storing the fuel vapors. When the internal combustion engines are driven, the adsorbed fuel vapors are eliminated from the adsorbents to the inlet ports of the combustion engines by inlet air. Thus, the adsorbents themselves are regenerated, and are used repeatedly.
As an adsorbent disposed in the inlet systems of the internal combustion engines, particulate activated carbon and fibrous activated carbon have been used in general. Recently, the pore diameter distribution of fibrous activated carbon has been controlled in order to devise techniques for reliably eliminating adsorbates from adsorbents and inhibiting the adsorption performance of adsorbents from degrading. For example, Japanese Examined Patent Publication (KOKOKU) No. 61-55,611 discloses such a fibrous activated carbon. In the fibrous activated carbon, the pore volume of the pores whose pore diameter is 120 xc3x85 (12 nm) or less is 0.61 cm3/g, the volume of the pores whose pore diameter is 120 xc3x85 (12 nm) or less and falls in a range of from 30 to 120 xc3x85 (from 3 to 12 nm) falls in a range of from 0.155 to 0.5 cm3/g, and the summed volume of the pores whose pore diameter falls in a range of from 30 to 120 xc3x85 (from 3 to 12 nm) occupies 17% by volume or more of the summed volume of the pores whose pore diameter is 120 xc3x85 (12 nm) or less.
However, when the fibrous activated carbon is used as the adsorbent, the adsorption performance might degrade as it is used for a long period of time under certain circumstances. Moreover, when the fibrous activated carbon is fixed on filters to use, the following problems might arise. In addition, the fibrous activated carbon might come off from the filters. The filters might exert increased resistance to the air flows in the inlet systems of internal combustion engines.
The present invention has been developed in view of the aforementioned circumstances. It is therefore an object of the present invention to provide an adsorbent for adsorbing fuel vapors, adsorbent which is less likely to degrade even when it is used for a long period of time. It is a further object of the present invention to provide an adsorbent for adsorbing fuel vapors, adsorbent which can be used reliably even when it is fixed on filters.
An adsorbent for adsorbing fuel vapors according to the present invention can achieve the aforementioned objects. The present adsorbent is for adsorbing fuel vapors, is to be disposed in an inlet system of an internal combustion engine, has pores, 50% by volume or more of the pores having a pore diameter falling in a range of from 1.4 to 2.8 nm, exhibits a unit pore volume of 0.3 mL or more with respect to 1 mL thereof, and is formed as a particle.
Since the present adsorbent has the pore diameter distribution and the unit pore volume, it exhibits high performance on adsorbing fuel vapors. Moreover, it shows high durability even when it is used for long period of time.
The present adsorbent can preferably comprise a porous carbonaceous material. Further, the porous carbonaceous material can preferably be activated carbon. Furthermore, in the present adsorbent, the particle can preferably comprise cylinder-shaped constituent particles whose diameter falls in a range of from 1.5 to 5.0 mm, or sphere-shaped or pulverized constituent particles whose particle diameter falls in a range of from 1.5 to 5.0 mm. Moreover, the present adsorbent can preferably exhibit a specific surface area of 1,500 m2/g or more.
In addition, the present adsorbent can preferably have a first mass enlarged by a factor of 50% or more when it is contacted with a first adsorbate, which includes air saturated with saturated benzene vapor, at a flow rate of 2.0 L/min. at standard temperature and pressure until the adsorbed mass of the adsorbent become constant, and has a second mass enlarged by a factor of 35% or more when it is contacted with a second adsorbate, in which air saturated with saturated benzene vapor is diluted by pure air with such a ratio that the volume of the air saturated with saturated benzene vapor with respect to the summed volume of the air saturated with saturated benzene vapor and the pure air is 1/10 by volume, at a flow rate of 2.0 L/min. at standard temperature and pressure until the adsorbed mass of the adsorbent become constant.
As having described so far, the present adsorbent is improved in terms of the performance on adsorbing fuel vapors as well as the durability. Accordingly, it is possible to suitably use the present adsorbent for adsorbing fuel vapors. Moreover, it is possible to form the present adsorbent into shapes which exert less resistance to air flows in inlet systems of internal combustion engines. Consequently, it is possible to reliably use the present adsorbent even when it is fixed on filters.